GDI+ - Cannot do anything with Gdiplus::Graphics (C++)

Question

I'm new to GDI+ and I've no idea what the problem is so I'll just post all of the code. I'm trying to simply draw an image but in my debugger I can see that graphics which I try to use in WM_PAINT is NULL. I've seen many people do pretty much the exact same thing which I'm trying to do so I'm pretty confused as to what's going on.

#include "stdafx.h"
#include "GUI.h"
#include <objidl.h>
#include <gdiplus.h>
#include <iostream>

using namespace Gdiplus;
#pragma comment (lib,"Gdiplus.lib")

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
WCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
WCHAR szWindowClass[MAX_LOADSTRING];            // the main window class name

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);


    // Initialize global strings
    LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadStringW(hInstance, IDC_GUI, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_GUI));

    MSG msg;

    // Main message loop:
    while (GetMessage(&msg, nullptr, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}

ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEXW wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_GUI));
    wcex.hCursor        = LoadCursor(nullptr, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = MAKEINTRESOURCEW(IDC_GUI);
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassExW(&wcex);
}

BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   hInst = hInstance; // Store instance handle in our global variable

   HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);

   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    PAINTSTRUCT ps;
    HDC hdc;
    switch (message)
    {
    case WM_COMMAND:
        {
            int wmId = LOWORD(wParam);
            // Parse the menu selections:
            switch (wmId)
            {
            case IDM_ABOUT:
                DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
                break;
            case IDM_EXIT:
                DestroyWindow(hWnd);
                break;
            default:
                return DefWindowProc(hWnd, message, wParam, lParam);
            }
        }
        break;
    case WM_PAINT:
        {
            hdc = BeginPaint(hWnd, &ps);
            Graphics graphics(hdc);
            Image image(L"C:\\light.png");
            graphics.DrawImage(&image, 0, 0);

            EndPaint(hWnd, &ps);
        }
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}

Answer 1

You are missing the calls to GdiplusStartup() and GdiplusShutdown().

From the reference of GdiplusStartup:

You must call GdiplusStartup before you create any GDI+ objects, and you must delete all of your GDI+ objects (or have them go out of scope) before you call GdiplusShutdown.

... and from GdiplusShutdown:

The GdiplusShutdown function cleans up resources used by Windows GDI+. Each call to GdiplusStartup should be paired with a call to GdiplusShutdown.

I'm using a RAII class like this to simplify the task:

class GdiPlusInit
{
public:
    GdiPlusInit()
    {
        Gdiplus::GdiplusStartupInput startupInput;
        Gdiplus::GdiplusStartup( &m_token, &startupInput, NULL );
        // NOTE: For brevity I omitted error handling, check function return value!
    }

    ~GdiPlusInit()
    {
        if( m_token )
            Gdiplus::GdiplusShutdown( m_token );
    }

    // Class is non-copyable.
    GdiPlusInit( const GdiPlusInit& ) = delete;
    GdiPlusInit& operator=( const GdiPlusInit& ) = delete;

private:
    ULONG_PTR m_token = 0;
};

Usage:

Create an instance of the class at the beginning of the scope where you want to use GDI+ functions (for performance reasons I wouldn't do that in a function that is called frequently). I usually create it as member variable of window classes or other classes that use GDI+ so clients of my code don't need to be told to initialize GDI+. It doesn't matter if clients already call GdiplusStartup() and GdiplusShutdown() on their own, because the calls can be nested, if they are properly paired.

In your case:

int APIENTRY wWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPWSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    GdiPlusInit gdiplus;  // calls GdiplusStartup() and stores the returned token

    // ... remaining code of your application ... 

    // When the scope ends, the destructor of GdiPlusInit calls GdiplusShutdown(),
    // passing the stored token.
}